Modem telecommunications networks are increasingly relying upon optical fibers and wavelength-division multiplexing (WDM). Silica optical fibers can transmit optical signals in optical bands near 1300 nm and 1550 nm over long distances with low loss and low chromatic dispersion. The digital transmission rate is primarily limited by the electronics at the transmitter and receiver ends, currently about 10 gigabits per second (Gb/s). The net data rate can be multiplied by wavelength division multiplexing in which the fiber carries multiple optical carrier wavelengths in one or the other of the previously mentioned bands. At the transmitter end, multiple laser diodes emitting light at respective ones of the multiple WDM wavelengths of number M have their outputs modulated by separate electronic data signals. The multiple optical signals are optically multiplexed together and then coupled onto the optical fiber. At the receiver end, the multiple WDM signals of number Mare optically demultiplexed to their constituent wavelengths. The individual optical signals are detected by a photodetector, typically a photodiode and most usually a PIN diode. A PIN diode has a large intrinsic region between its p- and n-type layers. The photodetector output is then conditioned and amplified for further processing as an electronic signal. The combination of photodetector and associated electronic circuitry is often referred to as an optical receiver. For a WDM receiver, the receiver function needs to be replicated for all the WDM wavelengths in what is referred to as a receiver array. WDM systems are being fielded with four wavelengths, that is, M=4. Sixteen wavelengths are being planned. The number M is expected to increase to up to about forty.
Great strides have been made in integrating the optical demultiplexer and photodetectors on a single integrated circuit chip, usually formed in compound semiconductors compatible with InP, for example, InGaAs. The receiver circuitry can also be integrated on the same chip, thus resulting in a moderately inexpensive, highly integrated receiver system.
However, the electronic circuitry required in a receiver array must satisfy several difficult requirements. To achieve long propagation spans, the optical signal level at the photodetector may be very small. The detection process may involve only a few photons. Thus, the receiver must accept low electronic input signals. The receiver circuitry must introduce a minimum of its own noise since low-level photodetector signals are already noisy. The receivers should operate at high data rates, a minimum of 2 Gb/s and preferably at least 10 Gb/s. For multiple receivers to be integrated onto a chip and using the same power supply lines, the cross talk between receivers, corresponding to different data channels, must be small.
Hence, in order to meet system requirements, the receiver array circuitry requires an advanced design.